Bridge circuit for determining the inverse of resistance

ABSTRACT

A BRIDGE CIRCUIT FOR USE IN SERVO SYSTEMS, PROVIDES A CURRENT FLOW WHICH IS A LINEAR FUNCTION OF THE INVERSE OF A RESISTANCE CONNECTED IN ONE OF THE BRIDGE ARMS. A SOURCE OF REFERENCE VOLTAGE IS CONNECTED IN ONE OF THE BRIDGE ARMS AND MEANS ARE PROVIDED TO MAINTAIN THE BRIDGE IN A NULL CONDITION AND TO CONTROL THE CURRENT IN THE BRIDGE CIRCUIT TO ACHIEVE THE DESIRED INVERSE RELATIONSHIP BETWEEN THE CURRENT AND RESISTNACE.

D. E. CATE BRIDGE CIRCUIT FOR DETERMINING THE INVERSE 0F RESISTANCEOriginal Filed July 11. 1966 2 Sheets-Sheet 1 IIIIIIIIL INVENTORDEXTER'. .4475

V ATTORNEYS 30, 1971 CAT-E Re. 27,103

BRIDGE CIRCUIT FOR DETERMINING THE INVERSE 0F RESISTANCE Original FiledJuly 11. 1966 Sheets-Sheet I TILIIEL 1 11: -5. I

,Szzm AMA INVENTOR Java/e5 6475 ATTORNEYS United States Patent cation;matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A bridge circuit for use in servo systems,provides a current flow which is a linear function of the inverse of aresistance connected in one of the bridge arms. A source of referencevoltage is connected in one of the bridge arms and means are provided tomaintain the bridge in a null condition and to control the current inthe bridge circuit to achieve the desired inverse relationship betweenthe current and resistance.

The present invention relates to a bridge circuit whose output isinverse to the resistance change of an electrical component such, forexample, as a thermistor, the resistance of which is a non-linearfunction of temperature.

The bridge circuit of the invention may include servo means to provideposition information which is a function of the ratio between areference voltage and a measured current, which position information maybe calibrated to provide an indication of a variable quantity to bemeasured; i.e., the varying resistance of a thermistor when subjected tochanging temperature conditions.

In accordance with the invention, I provide an electrical bridge circuitcomprising a plurality of bridge arms connected to each other to formanull[,] bridge, a resistor having a varying resistance value and beingconnected in one of said bridge arms, and feed-back means formaintaining a null condition in said bridge, characterized in that forproviding a current flovv which is a linear function of the inverse ofsaid varying resistance of said resistor, there is provided a source ofreference voltage which is connected in said one of said bridge arms.

Objects and advantages of the invention will be apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram of a bridge circuit utilized in a servosystem;

FIG. 2 is a modification of the bridge circuit of FIG. 1;

FIG. 3 is another modification of the bridge circuit of FIG. 1;

FIG. 4 is still another modification of the bridge ciredit of FIG. 1;

FIG. 5 is a circuit diagram of a bridge circuit illustrating the basicprinciple of the present invention;

FIG. 6 is a modification of the bridge circuit of FIG. 5 including anull maintaining branch; and

FIG. 7 is a circuit diagram of an embodiment of the bridge circuit ofthe present invention.-

Bridge circuits commonly used in servo feedback instrunients where theresistance is the measured variable may place great demands on thefeedback element. If

"Ice

resistance of the lead wires is significant with respect to the measuredresistance, a third lead must be used, and equal currents maintained inthe two halves of the bridge (see FIG. 1). As is well known in the art,to maintain a bridge balanced, or in null position, it is necessary toutilize a feed-back element. The feed-back element may take the formshown in FIGS. 2 to 4 which will hereinafter be described.

In FIG. 1, resistors R R R and R each form one arm of the bridge. Inseries with resistors R and R there is connected, for example, a sensorwhich may comprise a thermistor whose equivalent resistance in R Thelead resistance of the thermistor is R and if the resistance of thethermistor lead is significant with respect to the resistance of thethermistor to be measured, a third resistance R is used to maintainequal currents in the two halves of the bridge when the latter isbalanced. The current flowing between terminals 10 and 11 in FIG. 1should desirably be related inversely to the resistance change of, forexample, the thermistor. Also, by means of the feed-back element, theoutput terminals E and E should be maintained zero for all changes inresistance of the thermistor.

The bridge circuit of FIGS. 2, 3 and 4 are similar to that of FIG. 1excepting that in FIG. 2, resistor R is replaced by a variableresistance P in FIG. 3 resistor R is replaced by resistance P and inFIG. 4 resistor R is replaced by variable resistors P and P In FIG. 2,feed-back is accomplished by (1) a variable resistance R; in themeasurement arm whose variation in resistance is complementary to thechange in resistance of the thermistor. In FIG. 3, the variableresistance P is placed in the arm of the bridge opposite to thatillustrated in FIG. 2, the variation of resistor P being directlyrelated to the variation in resistance of the thermistor. In FIG. 4, twoganged variable resistors P and P are utilized, P maintaining equalcurrent in both halves of the bridge, variable resistor P serving tobalance the bridge to provide across terminal E and E a zero output. Ifthe measured resistance is non-linear function of the implied variable(as resistance in a thermistor is a non-linear function of temperature)the feedback must also be a non-linear function to give a linear output.If the non-linear input varies over a wide range, the gain of the servomay change by an intolerable ratio over the space of the instrumentcalibration.

Where a thermistor is used to measure temperature over a span of 50 F.or more, the measurement schemes described above result in the followingdeficiencies.

A variable resistance obtained from a rheostat as shown in FIGS. 2 and 3introduces contact noise directly as feedback. Change in slope of theresistance vs. temperature curve is introduced directly as gain change.A non-linear rheostat, or a shunted rheostat is required.

Ganged potentiometers, as shown in FIG. '4, may create contact noiseoutside the bridge in the supply circuit and in the amplifier inputcircuit. Furthermore, the use of a pair of ganged non-linear pots isundesirable from an economic standpoint.

The foregoing problems are avoided, in accordance with the invention, byintroducing a fixed voltage into one of the arms of a conventionalbridge of the type shown, for example, in FIG. 1. By introducing suchfixed voltage it is possible to determine the resistance of thethermistor by determining the current flowing in the bridge withoutresorting to prior art feed-back elements. The reason for this will beevident from the following analysis.

The state of the art in solid state operational amplifiers now makes thefollowing measurement scheme desirable from an economics standpoint,where the measured variable is, or can be made nearly like the functionwhere K is a constant, and R is a value of resistance. In the case of athermistor, a padding resistance, R can be added in series with thethermistor to produce a non-linearity (in the form of an S curve) of afew percent of span.

I have determined that by introducing a fixed voltage in one arm of aconventional bridge, it is possible to obtain a measuring circuit whichreads the inverse of resistance. This may be accomplished by varying thecurrent through the bridge until a null is reached, at which point thetwo halves of the bridge have equal current. Such variation of currentmay be obtained by utilizing the arrangement of FIG. 6, hereinafterdescribed. First, however, with reference to FIG. 5, like parts havebeen identified with the same reference characters utilized, forexample, in FIG. 1.

Assume E E =0, i.e., the bridge is at null. If R =R then bridge currentI must flow equally into R and R for E1-E3 to equal 0. We can writeequations:

1 1 1 [5 11 5 x ref ref] 14 x ref Rx rel and Current is therefore alinear function of the inverse of R By utilizing a fixed referencevoltage a balanced bridge with lead-wire compensation has been achieved,with variable current as the rebalance quantity. To use this bridge wemust:

(1) Maintainnull.

(2) Determine the current.

To maintain the null, the quantity 'E -E may be fed to an operationalamplifier having differential input (FIG. 6). The output is then used asbridge supply current. lnput polarity is connected to give negativefeedback. Provided the gain of the amplifier is sufiiciently high, E -Eis negligibly small and bridge balance 15 maintained.

where E is the reference voltage, R the resistance of the thermistor, Rthe resistance value of one of the arms of the bridge, and R is theresistance of the thermistor lead.

To measure the output current I can, of course, use a conventionalpotentiometric servo system and read the voltage drop in ,a fixedresistance inserted in the mph- 4 fier output circuit. Closer inspectionreveals, however, that it is preferable to obtain the ratio of outputcurrent to the reference, E as follows.

Equation 1 was:

I 5 2 rel Substituting in 2 i l R, R,

We now see that 1/ R,, which is of interest as a nearly linear functionof T, is directly proportional to r, and is also independent of BEquation 3 may be reduced to circuitry very simply as shown in FIG. 7,where the potentiometer P has a ratio 1' for B t/B t- P is driven by theservo amplifier until it sees null, at which the mechanical output is anindication of 'Pot ratio We now have, for servo null:

[% 2 ref I i 2= M which is the same as Equation 3 SUMMARY The circuit ofFIG. 7 has the following ch u'acteristics:

(1) Mechanical output, a direct function of r, is a measure of 1/ R (2)Serve gain is as linear as the function r.

(3) E is non-critical, affecting only the gain of the system.

(4) Voltage across the unknown, R,, is a constant, which may be ofinterest where insulation leakage in the lead wires is sensitive tovoltage magnitude, or polarity.

The foregoing disclosure relates only to preferred embodiments of theinvention which is intended to include all changes and modifications ofthe examples described within the scope of the invention as set forth inthe appended claims.

What is claimed is:

1. In a servo system, a bridge circuit for providing a current flowwhich is a linear function of the inverse of a resistance, said bridgecircuit comprising a plurality of bridge arms, connected to each otherto form a null bridge, a resistor connected in one of said bridge armsand having a resistance, a source of reference voltage connected inseries in one of said bridge arms, means connected to said bridgecircuit for maintaining said bridge in a null condition, said nullmaintaining means comprising means connected to said bridge circuit forcontrolling the current in said bridge circuit, said bridge circuitthereby having a current flow therein which is substantially a linearfunction of the inverse of the resistance of said resistance.

2. The combination as defined in claim 1, wherein said null maintainingmeans includes an operational signal flow.

3. The combination as defined in claim 1, further comprising apotentiometer connected in shunt across said source of referencevoltage, and second null maintaining means connected to said bridge forpositioning said potentiometer.

4 The combination as defined in claim 3, wherein said second nullindicating means comprises servo amplifier means having a pair of inputterminals, one of said pair of input terminals being connected to apoint on said bridge and the other of said pair of input terminals beingconnected to said potentiometer, said second null maintaining meanscomprising a servo motor connected to the output of said servo amplifiermeans.

5. The combination as defined in claim 4, also comprising mechanicalmeans for coupling said servo motor to said potentiometer.

6. The combination as defined in claim 1, wherein corresponding ones ofthe arms of said bridge circuit are connected to each other atconnecting points, said current control means comprising an operationalamplifier having a differential input connected to two connecting pointsof said bridge circuit and an output connected to a third connectingpoint of said bridge circuit, said operational amplifier having asubstantially high gain and providing a negative feedback betweenconnecting points of said bridge circuit, and further comprising meansfor determining the relationship between said reference voltage and thecurrent in said bridge circuit comprising potentiometer means connectedcross said source of reference voltage and including a movable slidemember for varying the resistance thereof and thereby providing a ratioof potentiometer voltage to said reference voltage, a servo amplifierhaving an input connected to the third connecting point of said bridgecircuit, an input connected to the slide member of said potentiometermeans and an output, and a servo motor having an input electricallyconnected to the output of said servo amplifier and being mechanicallycoupled to the slide member of said potentiometer means for moving saidslide member as a direct function of said ratio and indicating theinverse of said resistance.

7. A bridge circuit for providing a current flow which is a linearfunction of the inverse of a resistance, said bridge circuit comprisinga plurality of bridge arms connected to each other to form a nullbridge, a resistor connected in one of said bridge arms and having aresistance, a source of reference voltage connected in series in saidone of said bridge arms, null maintaining means connected to said bridgecircuit for maintaining said bridge in null condition, said nullmaintaining means comprising current control means connected to saidbridge circuit for controlling the current in said bridge circuit saidbridge circuit having a current flow therein which is a linear functionof the inverse of the resistance of said resistor.

8. A bridge circuit as defined in claim 7, wherein said nullifying meansincludes an operational amplifier having a diflerential input, saidamplifier having a pair of inputs connected to two points on saidbridge, said amplifier having its output connected to a third point onsaid bridge, said third point being between said two points.

9. A bridge circuit as defined in claim 7, further including apotentiometer connected in shunt across said source of referencevoltage.

10. A bridge circuit for producing an output value which is a linearfunction of the inverse of a resistance to be measured, said bridgecircuit comprising a plurality of bridge arms connected to each other toform a null bridge, a resistor connected to one of said bridge arms andhaving a variable resistance, a source of voltage connected in series inone of said bridge arms, null maintaining means connected to said bridgecircuit for maintaining said bridge in null condition, said nullmaintaining means comprising current control means connected to saidbridge circuit for controlling the current in said bridge circuitindependently of the impedances of said bridge arms to produce said nullcondition, and means connected to the bridge circuit to compare thevoltage of said source and a voltage drop in another arm of said bridgecircuit to produce said output value.

11. A bridge circuit for producing an output value which is a linearfunction of the inverse of a resistance to be measured, said bridgecircuit comprising a plurality of resistive bridge arms connected toeach other to form a null bridge, a resistor connected to one of saidarms and having a variable resistance, a source of voltage connected inseries in said one of said bridge arms, null maintaining means connectedto said bridge circuit for maintaining said bridge in null condition,said null means comprising current control means connected to saidbridge circuit for controlling the current in said bridge circuit in atleast a portion of said one arm independently of the resistances of saidbridge arms to produce said null condition, whereby the ratio of thevoltage of said source to a voltage drop in another arm of said bridgecircuit varies inversely with the resistance of said resistor, andoutput means connected to the bridge circuit for producing said outputvalue proportional to said ratio.

12. A circuit for producing an output value that varies as a linearfunction of the inverse of a resistance to be measured, said circuitcomprising a bridge circuit, means connecting said resistance in seriesin one arm of said bridge circuit, means for producing a voltageconnected in series with said resistance in said one arm of said bridgecircuit, and amplifier means having a pair of input terminals connectedbetween a pair of opposite terminals of said bridge circuit, meansconnecting the output of said amplifier to said bridge circuit toproduce a current flow in at least a portion thereof for adjusting saidbridge circuit to a null balance, whereby the ratio between said voltageand the voltage drop of another arm of said bridge circuit is inverselyproportional to the resistance to be measured, and output circuit meansconnected to said bridge circuit for producing said output valueproportional to said ratio.

13. The bridge circuit according to claim 11, wherein said output meanscomprise means for comparing said voltage and voltage drop to producesaid output value.

14. The bridge circuit according to claim 12, further comprising lineresistances each connected with one end in two difierent bridge arms andwith the other end to said resistance to be measured.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,349,321 10/1967 Arskey 323-75N 3,443,215 5/1969Bradley 323-75NX 3,457,493 7/1969 Shoemaker et a1. 323--75NX 3,111,62011/1963 Baker 32375 3,247,703 4/1966 Burk 323-45 FOREIGN PATENTS 671,9515/1952 Great Britain 31818(20.745)

BENJAMIN DOBECK, Primary Examiner US. Cl. X.R.

